The present invention relates generally to radar signal processing, and more particularly, to a signal processor and signal processing method that implements monopulse thresholding to improve azimuth resolution and provide enhanced radar imagery.
A method that uses signals derived from the difference channel to improve azimuth resolution, known as monopulse resolution improvement, was previously investigated by the assignee of the present invention. The study found that the monopulse resolution improvement method separated targets only under conditions where the targets have a high signal-to noise ratio and are already visually separable. It was concluded that the monopulse resolution improvement method was unsuited for image sharpening, because even when targets are separable by the monopulse resolution improvement method, the method produced false targets from sidelobes.
The monopulse resolution improvement method was implemented using the following steps. Let SUM(n) be the sum channel signal for an image cell n. Let .vertline.DIFF(n).vertline. be the magnitude of the difference channel signal for an image cell n. Let k be a known constant multiplier. Then for each image cell n,
IF SUM(n)&gt;k .vertline.DIFF(n).vertline.,
THEN use SUM(n)--k .vertline.DIFF(n).vertline. as the image signal for that cell, PA1 ELSE, set the image signal for the cell to 0.
U.S. Pat. No. 4,978,680, issued Dec. 18, 1990, uses monopulse radar returns to achieve resolution improvement. This patent uses sum and difference channel data to form discriminants, and then using the discriminants, it repositions the sum channel signal data to the measured azimuth value. This technique does well in sharpening discrete targets but has problems with distributed clutter and targets. It also requires the radar to receive in two channels instead of one. This technique is subject to positioning the returns in the wrong location in low signal-to-noise ratio conditions or in multiple targets in the beam conditions.
Consequently, heretofore monopulse signals were used to provide azimuth beam sharpening and required that the sum and azimuth difference channels be received in the radar separately in order to use the data to perform the beam sharpening. Furthermore, monopulse signals were used to provide beam sharpening using the sum and difference dam to generate a discriminant in order to perform the beam sharpening.
It is therefore an objective of the present invention to provide for a signal processor and processing method that reduces the need to receive two channels by generating a pseudo azimuth difference channel information from the sum channel information. It is a further objective of the present invention to provide for a signal processor and signal processing method that uses the pseudo difference channel data as a threshold on the sum channel data to perform the beam sharpening.